The present invention relates to a highly efficient semiconductor laser unit having appropriate temperature characteristics used for the light source of a laser printer or bar code reader.
The semiconductor laser unit is fundamentally composed of an active layer and cladding layers, in which the active layer is interposed between the cladding layers. Various semiconductor materials are used for the semiconductor laser unit. For example, a red semiconductor laser for oscillating a beam of light of short wavelength is usually composed of AlGaInP material. In this case, a beam of light of short wavelength is defined as a beam of light of which the wavelength is shorter than the wavelength of an infrared ray which is in the oscillating band of a common semiconductor laser.
However, in the case of this AlGaInP material, it is not possible to provide a large band gap between the cladding and active layers. Therefore, a problems is caused in which carriers, especially electrons, overflow from the active layer to the cladding layers so that the laser characteristics are deteriorated. That is, the luminous efficiency of the laser is low due to the overflow of carriers, and further the temperature characteristics are deteriorated.
As a means for solving the above problems, a multiquantum barrier structure has been proposed. The multiquantum barrier structure is a superlattice in which well layers and barrier layers are alternately laminated. Due to the changes in potential and effective mass, the multiquantum barrier structure is capable of reflecting electrons of high energy compared with the barrier layer composing the multiquantum barrier structure. When the multiquantum barrier layer is provided between the active and cladding layers or in the cladding layers close to the active layer, the electrons in the active layer can be prevented from overflowing, so that the laser characteristics such as temperature characteristics and luminous efficiency can be improved, which is disclosed and published in the following documents:
Official gazette of Japanese Unexamined Patent Publication No. 4-114486; "Design of multiquantum barrier (MQB) and experimental verification of reflecting effect of electronic waves" by Takagi and et al. on page 527 to 535 of Vol. J74-C-I of theses of C-I of Electronic Information Communication Society published in December of 1991; "Experimental verification concerning MQB effect of AlGaInP visible radiation laser" by Arimoto et al. disclosed in 19a-V-5 in the fifty third lecture meeting of the Applied Physics Society held in autumn of 1992; and "Superposed Multiquantum Barriers for InGaAlP Hetero-junctions" by Furuya et disclosed in IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 28, NO 10, OCT. 1992, pp 1977-1982.
However, in the case of the multiquantum barrier structure of the prior art, consideration is given only to the reflection of electrons which occurs at a position close to point .GAMMA. which is one of the symmetrical points in the reciprocal lattice space, and consideration is not given to the reflection of electrons which occurs at a position close to point X or L which is another primary symmetrical point. In this connection, the reciprocal lattice space is a space for expressing a crystal structure, and the substantial lattice of the crystal structure and the reciprocal lattice in the reciprocal lattice space can be converted to each other in accordance with a predetermined converting expression. When the actions of electrons in the crystal and those of holes are described using this reciprocal lattice space, it becomes easy to treat them physically. In the case where various lattices are three-dimensionally expressed using this reciprocal lattice space, symmetrical points are referred to as .GAMMA., L and X points.
In the conventional multiquantum barrier structure described above, electrons at a position close to .GAMMA. point are effectively reflected, so that they are prevented from overflowing to the cladding layers, however, electrons at a position close to X or L point are not reflected, so that they overflow. Consequently, electrons having high energy move to X or L point, the energy level of which is low, and they overflow to the cladding layers.
In FIG. 10, is shown a band structure of points .GAMMA. and X of the multiquantum barrier proposed in the above mentioned "Design of multiquantum barrier (MQB) and experimental verification of reflecting effect of electronic waves" by Takagi and et al. on page 527 to 535 of Vol. J74-C-I of theses of C-I of Electronic Information Communication Society published in December of 1991. In this structure, the cladding layers 51 and the barrier layer 53 of the multiquantum barrier 52 are composed of the composition of (Al.sub.0.7 Ga.sub.0.3)InP, so that the energy level of point .GAMMA. and that of point X are permitted to be approximately the same. In this case, the well layer 54 is composed of GaInP. In this connection, numeral 55 is an active layer of GaInP.
In FIG. 1, an example of the reflectivity of electrons is shown when the electrons are reflected on the multiquantum barrier having the band structure of .GAMMA. point described above. In this case, U.sub.0 is 115 meV, the well layer thickness is 6 atomic layers, and the barrier layer thickness is 5 atomic layers. As can be seen in the graph of FIG. 11, when the multiquantum barrier is used, the reflectivity is high even in the case of electrons of relatively high energy, so that the electrons are prevented from overflowing. However, electrons move from a position close to point .GAMMA. on the active layer 55 to a position close to point X on the first barrier layer 53a (shown in FIG. 10) of the multiquantum barrier, and the electrons are reflected by the reflectivity shown in FIG. 12. As can be seen from the graph shown in FIG. 12, the reflectivity of electrons at the position close to point X is low in an energy region (in a region close to 120 meV in the example shown in FIG. 12), and electrons in this region pass through the multiquantum barrier 52 and overflow onto the cladding layers 51. As a result, the electrons overflow outside from the active layer 55.
As described above, in the multiquantum barrier structure of the prior art, carriers are not sufficiently suppressed from overflowing, and the most appropriate multiquantum barrier structure can not be provided.
It is an object of the present invention to provide a highly efficient semiconductor laser unit having excellent temperature characteristics, in which electrons or holes are suppressed from overflowing from the active layer to the cladding layers while the threshold of current density is maintained low.